Purple Wedding
The Purple Wedding is the name by which the wedding feast of King Joffrey Baratheon and Margaery Tyrell came to be remembered, on account of the king's death by poison at his own celebration. Held in King's Landing not long after the Battle of the Blackwater and the Red Wedding, it marked the sudden and violent end of one of the most hated kings ever to sit the Iron Throne.
The marriage united House Lannister and House Tyrell, the two great powers whose alliance had saved the capital at the Blackwater, and was meant to crown their victory in the War of the Five Kings with a lavish public spectacle.
The Feast
The wedding was celebrated with extravagant pomp -- a feast of seventy-seven courses, mummers, jugglers, and a cruel pageant of dwarfs mocking the dead kings of the war, which Joffrey found vastly amusing and which humiliated his uncle Tyrion Lannister. Throughout the day the king's malice was on full display as he taunted and degraded Tyrion before the assembled court.
As the feast drew on, Joffrey drank from a cup of wine and abruptly began to choke, clawing at his throat. Within moments he was dead, his face dark and swollen from the poison. In the chaos, Cersei Lannister, maddened with grief and rage, accused her brother Tyrion -- who had been made to serve as the king's cupbearer in his humiliation -- of murdering the king, and had him seized.
Aftermath
Tyrion was arrested and put on trial for regicide, though he was innocent of the deed. The true authors of the poisoning worked from the shadows, and the boy Tommen, Joffrey's younger brother, was raised to the throne in his place. The death set in motion a fresh round of intrigue, accusation, and bloodshed within King's Landing, and deepened the deadly enmity between Cersei and Tyrion.
Significance
The Purple Wedding removed the tyrant Joffrey from the board and threw the Lannister regime into turmoil at the height of its triumph. By casting suspicion on Tyrion Lannister, it shattered what remained of his standing and drove him toward exile, while the question of who truly poisoned the king became one of the great intrigues of the court.